In the above technical field, patent literature 1 discloses a technique in which if a tracking object is hidden by a shielding object and appears again, the same identification number is added to the tracking object. Patent literature 2 discloses a technique in which if a tracking vehicle is hidden by a shielding object, the predicted position of the tracking vehicle is displayed on a monitor. Furthermore, patent literature 3 discloses a technique of giving an alarm by predicting the risk that an object in a blind spot of a driver runs out.